We have demonstrated that indium-111 8-hydroxyquinoline provides a substantially improved radioisotopic label for platelets. Indium-111 is a short half-life gamma-emitting nuclide with a high photon yield; therefore in addition to statistically improved kinetic data, it should be possible to do precise studies of the in vivo distribution and fate of platelets. Our specific objectives in this program are 1) to optimize the conditions of labeling autologous platelets with indium-111 for studies in human subjects; 2) to standardize techniques for precise kinetic studies and quantitative measurements of the spatial and temporal distribution of indium-111 labeled platelets in man; 3) to study the survival and spatial and temporal distribution patterns of indium-111 labeled platelets in human health and selected diseases. We will begin our studies using indium-111 platelets in human disease with a reassessment of the role of the spleen in idiopathic (immune) thrombocytopenic purpura and of platelet kinetics in patients with homocystinuria.